1. Field of the Invention
The present invention relates to a technique that can control a tip-base metal distance in an arc welding system including a welding power source having a constant-voltage characteristic.
2. Description of the Related Art
In arc welding, it is important to maintain a fixed distance between a tip at an end of a welding torch and a base metal. That is, if the tip-base metal distance changes during welding, the penetration depth and the welding bead width change, or sputters, blowholes, etc. are caused. This makes welding unstable and lowers the welding quality. Particularly when automatic welding is performed with an arc welding system using a welding robot, if the tip-base metal distance changes, shieldability of shield gas is sometimes reduced, or the tip and the base metal, or a shield gas nozzle and the base metal sometimes touch each other. Hence, it is necessary to perform control so that a fixed tip-base metal distance is maintained.
In a general arc welding system using a welding power source having a constant-voltage characteristic, a copying operation (copying) is performed to maintain a fixed tip-base metal distance. The copying operation automatically corrects the position of a welding torch in an upward direction or a downward direction by causing the actual welding current value to coincide with a target current value. The constant-voltage characteristic refers to a characteristic in that the welding voltage value is fixed, regardless of increase or decrease in actual welding current value. A specific description will be given below of a flow of a procedure adopted when a tip-base metal distance control method of the related art is applied to an arc welding system, with reference to FIG. 13.
In the related art, it is first determined in Step S101 whether or not copying of the arc welding system is effective. When copying is effective (Yes in Step S101), in Step S102, actual welding current values detected by a sensor or the like are subjected to A/D conversion and sampling. In contrast, when copying is not effective (No in Step S101), the procedure is finished.
Next, in Step S103, it is determined whether or not sampling for a predetermined period is completed. When the sampling is completed (Yes in Step S103), an average actual welding current value is calculated from the sampled actual welding current values in Step S104. In contrast, when the sampling is not completed (No in Step S103), the procedure returns to Step S102.
Next, in Step S105, it is determined whether or not a set welding current value is larger than the average actual welding current value. When the set welding current value is larger than the average actual welding current value (Yes in Step S105), in Step S107, the actual welding current value is increased by correcting the position of the welding torch in the downward direction, and the procedure returns to Step S101. In contrast, when the set welding current value is not larger than the average actual welding current value (No in Step S105), the procedure proceeds to Step S106.
In Step S106, it is determined whether or not the set welding current value is smaller than the average actual welding current value. When the set welding current value is smaller than the average actual welding current value (Yes in Step S106), in Step S108, the actual welding current value is increased by correcting the position of the welding torch in the upward direction, and the procedure returns to Step S101. In contrast, when the set welding current value is not smaller than the average actual welding current value (No in Step S106), the procedure proceeds to Step S101.
Besides this copying operation of the related art, for example, Japanese Unexamined Patent Application Publication No. 11-58012 discloses a technique that indirectly controls the tip-base metal distance by controlling a wire extension length. That is, in this technique, resistances of wires having different extension lengths are measured and stored as data beforehand, the resistance of a wire in actual welding is calculated from the current and voltage values of the wire, and the calculated resistance is compared with the prestored resistances, whereby the extension length of the wire is estimated.
However, in the above-described copying operation of the related art, the change in set welding voltage value (set value of welding voltage) adversely affects the tip-base metal distance. That is, in arc welding using the welding power source having the constant-voltage characteristic, the arc length changes almost in proportion to the set welding voltage value. Further, if the arc length changes, the wire extension length at an end of the tip changes and the resistance of the wire changes. Hence, the actual welding current value also changes.
For this reason, for example, when arc welding is performed while the wire feeding speed and the tip-base metal distance are fixed, if the set welding voltage value decreases, the arc length decreases, and the actual welding current value also decreases. When the actual welding current value thus decreases, the above-described copying operation performs control so as to make the actual welding current value closer to the set welding current value. That is, the resistance of the wire is decreased and the actual welding current value is increased by correcting the position of the welding torch in the downward direction so as to decrease the wire extension length. Such correction of the position of the welding torch is performed until the actual welding current value coincides with the set welding current value. Therefore, in the copying operation of the related art, if the set welding voltage value changes, the tip-base metal distance also changes.
The set welding voltage value in arc welding is appropriately changed in correspondence with the change of the shape of the base metal and the welding position. Therefore, in the copying operation of the related art that does not consider the influence of the set welding voltage value, the tip-base metal distance cannot be accurately controlled to be fixed at an actual arc welding site. Moreover, devices, such as a sensor for current detection and an A/D converter for current conversion, are sometimes used to detect the actual welding current value and input the actual welding current value to a control unit in the copying operation of the related art. However, these devices usually have errors of several percent to ten percent. Therefore, in the copying operation of the related art that needs to obtain an accurate actual welding current value, it is difficult to accurately control the tip-base metal distance to be fixed.
Similarly to the related art, the above-described Japanese Unexamined Patent Application Publication No. 11-58012 does also not consider the influence of the set welding voltage value, and cannot accurately control the tip-base metal distance to be fixed. In addition, while the wire and the base metal need to be short-circuited during welding in order to calculate the resistance of the wire in the invention of the publication, this invention is not applicable, for example, to spray transfer (droplet transfer in which the wire end is melted by arc heat and droplets smaller than the diameter of the wire fly in the arc and are transferred onto the base metal).